The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases.
The operation of a catalytic combustible gas sensor proceeds through electrical detection of the heat of reaction of a combustible gas on the oxidation catalyst, usually through a resistance change. The oxidation catalysts typically operate in a temperature above 300° C. to catalyze combustion of an analyte (for example, in the range of 350 to 600° C. temperature range for methane detection). Therefore, the sensor must sufficiently heat the sensing element through resistive heating. In a number of combustible gas sensors, the heating and detecting element are one and the same and composed of a platinum alloy because of its large temperature coefficient of resistance and associated large signal in target/analyte gas. The heating element may be a helical coil of fine wire or a planar meander formed into a hotplate or other similar physical form. The catalyst being heated often is an active metal catalyst dispersed upon a refractory catalyst substrate or support structure. Usually, the active metal is one or more noble metals such as palladium, platinum, rhodium, silver, and the like and the support structure is a refractory metal oxide including, for example, one or more oxides of aluminum, zirconium, titanium, silicon, cerium, tin, lanthanum and the like. The support structure may or may not have high surface area (that is, greater than 75 m2/g). Precursors for the support structure and the catalytic metal may, for example, be adhered to the heating element in one step or separate steps using, for example, thick film or ceramic slurry techniques. A catalytic metal salt precursor may, for example, be heated to decompose it to the desired dispersed active metal, metal alloy, and/or metal oxide.
As illustrated in FIGS. 1A and 1B, a number of conventional combustible gas sensors such as illustrated sensor 10 typically include an element such as a platinum heating element wire or coil 20 encased in a refractory (for example, alumina) bead 30, which is impregnated with a catalyst (for example, palladium or platinum) to form an active or sensing element, which is sometimes referred to as a pelement 40, pellistor, detector or sensing element. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987). Combustible gas sensors are also discussed generally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases, Heinemann, Exeter, 29 (1981).
Bead 30 will react to phenomena other than catalytic oxidation that can change its output (i.e., anything that changes the energy balance on the bead) and thereby create errors in the measurement of combustible gas concentration. Among these phenomena are changes in ambient temperature, humidity, and pressure.
To minimize the impact of secondary effects on sensor output, the rate of oxidation of the combustible gas may, for example, be measured in terms of the variation in resistance of sensing element or pelement 40 relative to a reference resistance embodied in an inactive, compensating element or pelement 50. The two resistances may, for example, be part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG. 1C. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement 50 are typically matched as closely as possible with active or sensing pelement 40. In a number of systems, compensating pelement 50 may, however, either carry no catalyst or carry an inactivated or poisoned catalyst. In general, changes in properties of compensating elements caused by changing ambient conditions are used to adjust or compensate for similar changes in the sensing element.
Catalytic combustible gas sensors are typically used for long periods of time over which deterioration of the sensing element or the like and malfunction of circuits may occur. A foreign material or contaminant such as an inhibiting material or a poisoning material (that is, a material which inhibits or poisons the catalyst of the sensing element) may, for example, be introduced to the sensing element. An inhibiting material typically will “burn off” over time, but a poisoning material permanently destroys catalytic activity of the sensing element. Inhibiting materials and poisoning materials are sometimes referred to herein collectively as “poisons” or “poisoning material.” In general, it is difficult to determine such an abnormal operational state or status of a combustible gas sensor without knowingly applying a test gas to the combustible gas sensor. In many cases, a detectible concentration of a combustible gas analyte in the ambient environment is a rare occurrence. Testing of the operational status of a combustible gas sensor typically includes the application of a test gas (for example, a gas including a known concentration of the analyte or a simulant thereof to which the combustible gas sensor is similarly responsive) to the sensor. Periodic testing using a combustible gas may, however, be difficult, time consuming and expensive.
For decades, combustible gas sensor designers have been perplexed with the problems of contamination and/or degradation of their catalyst structures. Sulfur-containing compounds (inhibitors) have been known to target and inhibit the catalyst structures. Filtering techniques are generally used to prevent their passage into the structure. If they do enter the structure, they are bound until a sufficient level of heat is applied to promote their release or decomposition. Volatile silicon/organosilicon compounds (poisons) are also known to cause significant issues with catalytic structures as they are permanently retained, and eventually result in the total inactivity of the catalyst. Further, high levels of hydrocarbons can also deposit incomplete and/or secondary byproducts such as carbon within the structure. Lead compounds, organophosphates and halogenated hydrocarbons are also known to poison/inhibit catalysts used in combustible gas sensors.
Manufacturers may add a layer of inhibitor/poison absorbing material outside of the supported catalyst of a sensing element as well as a compensating element. However, exposure to a sufficient amount of inhibitor/poison can still render the catalyst inactive. Moreover, increasing the mass of the sensing/compensating element increases the power requirements of the sensor, which may be undesirable, particularly in the case of a portable or other combustible gas sensor in which battery power is used.
Moreover, an inhibited or poisoned sensing element may go undetected by, for example, high sensitivity bridge and other circuits used in combustible gas sensors. Users have long reported cases where their catalytic sensors are reading zero (that is, the bridge circuitry is balanced), yet the sensors show little response to gas challenges. A notable example of this effect occurs when an organosilicon vapor such as hexamethyldisiloxane (HMDS) is introduced to the sensor. The HMDS will indiscriminately diffuse into the sensor housing and surroundings, adsorb onto the surface of the detector and/or compensator, and oxidize into a layer of silica (silicon dioxide or SiO2). Since both elements are typically operated at similar temperatures, silicone deposition occurs at an equal rate, keeping the bridge in balance. Unfortunately, this renders the elements permanently inactive. Indeed, some manufacturers use this poisoning process to manufacture compensating elements or compensators for combustible gas sensors.
A number of methods and systems have been developed to sense inhibition/poisoning in a catalytic sensing element with only limited success. Recent advancements include, for example, methods utilizing additional or alternative electrical properties of the catalytic structure such as reactance to analyze one or more variables related to reactance. While such systems and methodologies are able to diagnose the deposition of poisons and inhibitors within the structure of an element for a combustible gas sensor, such systems and methodologies find limited success in detecting the deposition or formation of surface materials which can also block the sensing elements ability to interact with the target gas. It remains desirable to develop diagnostic systems and methods for catalytic sensors and structures to detect inhibition/poisoning.